Digital radiography is increasingly accepted as an alternative to film-based imaging technologies that rely on photosensitive film layers to capture radiation exposure and thus to produce and store an image of a subject's internal physical features. With digital radiography, the radiation image exposures captured on radiation-sensitive layers are converted, pixel by pixel, to electronic image data which is then stored in memory circuitry for subsequent read-out and display on suitable electronic image display devices.
The perspective view of FIG. 1 shows a partial cutaway view of a small edge portion of an indirect DR panel 10. A scintillator screen 12 responds to incident x-ray radiation by generating visible light that is, in turn, detected by a flat panel detector (FPD) 20. Detector 20 has a two-dimensional array having many thousands of radiation sensitive pixels 24 that are arranged in a matrix of rows and columns and are connected to readout element 25. As shown at enlarged section E, each pixel 24 has one or more photosensors 22, such as a PIN diode or other light-sensitive component, and an associated switch element 26 of some type, such as one or more thin film transistors, or TFTs. To read out image information from the panel, each row of pixels 24 is selected sequentially and the corresponding pixel on each column is connected in its turn to a charge amplifier (not shown). The outputs of the charge amplifiers from each column are then applied to other circuitry that generates digitized image data that then can be stored and suitably image-processed as needed for subsequent storage and display.
Indirect DR imaging shows promise for providing improved diagnostic imaging performance with high levels of image quality. However, some drawbacks remain. Because scintillator materials respond to incident x-ray radiation by emitting light over a broad range of angles, there is some inherent amount of scattering in the indirect detection process. This reduces the optical efficiency of image formation due to loss of light, signal crosstalk, and related effects, and tends to degrade image quality.
For a better understanding of the optical coupling problem, it is helpful to consider components of DR panel 10 more closely and with particular attention to how light emitted at larger angles is handled. FIG. 2A shows a cross-sectional view of component layers of DR panel 10 having conventional fabrication. Scintillator screen 12 has a scintillator layer 16 formed on a support 14 that is highly transmissive to incident x-ray radiation. A protective overcoat layer 18 may be provided for scintillator layer 16. Detector 20 may comprise a PIN diode as photosensor 22, with a p-doped layer 38, an I-layer (intrinsic or undoped layer) 36, and an n-doped layer 34 formed on a metal layer 32 which is itself supported by a substrate 30, typically of glass. A transparent ITO (Indium-Tin Oxide) layer 40 provides conductive traces. A passivation layer 42 then adds insulation and surface uniformity.
Scintillator layer 16 material responds to incident x-ray R by emitting photons toward photosensor 22, but over a broad range of angles, including angles at which the emitted light is effectively wasted due to total internal reflection (TIR) effects within the scintillator layer 16 or, if provided, overcoat layer 18. Using the model arrangement of FIG. 2A, as long as there is good optical coupling between scintillator screen 12 and detector 20, a sufficient amount of the emitted signal is directed toward photosensor 22. In FIG. 2A, overcoat layer 18 is illustrated to be in theoretical total contact with passivation layer 42, providing optimal coupling.
In practice, however, there is often poor optical coupling between scintillator screen 12 and detector 20. FIG. 2B shows an air gap 44 (dotted outline) between layer 18 of scintillator screen 12 and layer 42 of detector 20. For light at very small angles of incidence (relative to normal) with respect to passivation layer 42, the net effect of air gap 44 can be negligible. But for light at larger angles, air gap 44 causes a problem. When light is incident from a dense medium with a higher index of refraction n to a rare medium with a lower index of refraction n′ (e.g., n′=1.0 for air), total internal reflection may occur at the interface of the two media depending on the angle of incidence. With respect to FIG. 2B, TIR occurs at the interface between layer 18 and air gap 44, for incident light at or exceeding a critical angle θc defined by:
      θ    c    =            sin              -        1              ⁡          (              1        n            )      
For an overcoat layer of polymethyl methacrylate or PMMA, for example (with n=1.47), critical angle θc is approximately 43 degrees. This means that light at any angle greater than this value is reflected at the interface, rather than directed to photosensor 22. Some portion of this light is generally lost; another portion would be redirected to the wrong photosensor element as crosstalk. The net effects include lost efficiency and reduced spatial resolution (generally measured by the modulation transfer function, MTF).
Improved optical coupling between the scintillator screen and the detector would help to boost efficiency and improve overall image quality accordingly. Generally, solutions that have been proposed for doing this have shown only limited success. Moreover, although some of the proposed solutions may indeed improve optical coupling, they achieve this at the cost of increased complexity and higher expense or inadvertently introduce other problems.
Among methods employed for improving optical coupling between the scintillator screen and the detector are the following, represented schematically in FIGS. 3A through 3F:
(i) Applying continuous pressure between the scintillator screen and the flat panel detector, thereby maintaining physical contact between these assemblies. This type of solution, shown by arrows in FIG. 3A, is defeated by the dimensional requirements of the DR panel. A typical indirect DR detector may be as large as 17×17 inches (289 square inches). It is difficult to maintain optical contact between a scintillator screen and a photodiode array throughout the whole detector area at this scale. Uniformity of optical contact is a must. Where an air gap occurs, the light transmission and the spatial resolution (MTF) would be significantly degraded.
(ii) Depositing a scintillator layer directly onto the photodiode array. FIG. 3B shows a deposition apparatus 50 for forming scintillator layer 12. This method assures physical contact, hence optical contact. However, this type of processing can be complex, may risk damage to the photodiode array and can be very expensive. Flat panel detector 20 is an expensive device and can cost many thousands of dollars, making it impractical to use as a “substrate” for deposition or coating of materials. Uniformity of deposition also presents an obstacle that makes this type of solution less than desirable.
(iii) Use of a fiber-optic plate (FOP) 52 between detector 20 and scintillator screen 12, as shown in FIG. 3C. FOP 52 is an optical device consisting of several millions of glass fibers 54, each a few micrometers in diameter, bonded in parallel to one other. Light from the radiation image is transmitted from the scintillator to the photodiode array through each fiber. A typical FOP is about 3 mm thick and the diameter of fibers 54 is about 6 um. Scintillator screen 12 is disposed on FOP 52, then this combination is pressed against detector 20. Air gaps 44 still may be present on either side of FOP 52. Although the FOP provides high-resolution imaging, it suffers a considerable light loss (about 37%). FOP transmittance is about 63% for Lambertian light at the wavelength of 0.55 um. In addition, air gaps 44 can still occur on either surface of FOP 52. This solution, therefore, also encounters the problems described in (i) and shown in FIG. 3A.
(iv) Depositing a scintillator layer directly onto the fiber-optic faceplate. FIG. 3D shows this hybrid solution. This solution reduces or eliminates air gaps 44 between scintillator screen 12 and FOP 52; however, there can still be an air gap problem at the other surface of FOP 52. This solution also suffers from low transmittance as at (iii).
(v) As in FIG. 3E, depositing a scintillator layer directly onto the fiber-optic faceplate as in (iv) and applying an optical adhesive 56 between the coated FOP 52 and detector 20. As with methods (iii) and (iv) just given, this method suffers from the inherently low transmittance of the fiber-optic faceplate.
(vi) As in FIG. 3F, insertion of a conventional optically transparent polymer layer 58 between scintillator screen 12 and detector 20. The optical polymer materials used for this purpose may be in the form of fluid, gel, thermoplastic material, or glue. Each of these optical polymers has accompanying problems. Optical fluids are the most convenient to apply. However, as true fluids, they require containment or will otherwise tend to flow out from the optical interface if unsealed. Optical gels are non-migrating and do not require containment seals. However, they are too soft to provide dimensional rigidity, and may swell with prolonged exposure or at elevated temperatures. Optical thermoplastics (such as elastomers and resins) include soft plastics that, when cured, provide some dimensional rigidity. However, an additional thermal or radiation process for curing is generally required; such processing can be risky for electronic components of detector 20. Optical glues exhibit similar problems as optical gels. It is also difficult to apply a uniform thickness of glue between the scintillator screen and the detector array. One solution for this problem, proposed in U.S. Pat. No. 5,506,409 to Yoshida et al. entitled “Radiation Detecting Device and the Manufacture Thereof”, is the use of spherical spacers to ensure the proper adhesive thickness. However, this requires a number of added steps for proper adhesion, with some complexity and risk of irregular spacer distribution.
Further difficulties occur in attempting to deposit a columnar scintillator such as CsI directly onto the photodiode array or the fiber-optic faceplate, as noted in (ii), (iv), and (v) above. A structureless layer (or seed layer) of finite thickness is first formed before a layer of CsI with columnar structure is formed. This structureless layer would tend to provide significant scattering of light before reaching the photodiode array or the fiber-optic faceplate and degrade the spatial resolution (MTF). Alternatively, a columnar scintillator is first directly deposited onto a substrate so that the structureless seed layer is formed between the substrate and the columnar layer, thus minimizing the light scattering effect. The resulting scintillator screen is then pressed against the photodiode array or the fiber-optic faceplate. However, as discussed in solution (i) above, an air gap would be very likely if optical contact is not maintained across the detector surface. This problem would be particularly acute with a large DR detector.
Still other approaches for boosting the light efficiency of DR panels focus more directly on scintillator materials design. For example, European Patent Application EP 1 024 374 entitled “Scintillator Panel and Radiation Image Sensor” by Homme et al. describes an improved substrate selection for the scintillator screen, with deposition of columnar CsI (cesium iodide) crystals on the substrate. The scintillator screen is then stuck in some manner against the detector array panel for imaging. As with the approaches listed in (i) to (iv) above, the solution offered by Homme et al. does not alleviate the problems caused by air gaps at the interface. Moreover, this solution does not indicate whether or not adhesives are used. Some types of adhesives, such as epoxies and others, contain compounds that are harmful to CsI crystal material.
As DR technology advances, new materials and fabrication techniques are being developed to lower the cost and improve the performance of the DR panel. One upcoming development that has particular value relates to fabrication of detector and scintillator layers on substrates that are increasingly more flexible. While this offers significant advantages for diagnostic imaging, such as in dental imaging, for example, the use of flexible substrates also presents considerable challenges. One of these challenges is in maintaining good optical coupling between the scintillator and detector components. Conventional techniques, as described earlier in (i)-(vi) and with reference to FIGS. 3A-3F, do not appear well-suited to meeting this challenge.
Thus, it can be seen that, while there have been a number of attempts to improve DR panel efficiency, there is still room for improvement. Solutions that reduce or eliminate TIR at the scintillator/detector interface without an elaborate number of steps and using materials appropriate for the scintillator or detector components would be particularly helpful. Moreover, solutions that are adaptable for future, more flexible substrates are needed.